Passenger vehicles include a powertrain that is comprised of an engine, multi-speed transmission, and a differential or final drive train. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. Automatic multi-speed transmissions improve the operating performance and fuel economy of the vehicle.
The benefits of three and four speed automatic-shifting transmissions have caused their popularity among the motoring public to significantly increase over the last few decades. Moreover, the demand for having vehicles equipped with an automatic transmission has not been curtailed by the demands for having vehicles with more fuel-efficient power sources. Users of hybrid (electro-mechanical) vehicles, for example, still desire vehicles equipped with an automatic transmission. However, the transmission-related packaging requirements for hybrid vehicles are more demanding than those for non-hybrids since hybrids require the use of at least one electric motor to supply power to the automatic transmission. The presence of the electric motor significantly reduces the amount of packaging space available for the transmission and its necessary components. Design demands, therefore, aim for fitting the electro-mechanical transmission within the smallest possible packaging space.
Integral to any automatic planetary transmission is at least one planetary gear arrangement consisting of a sun gear, planetary carrier with pinion gears, and a ring gear. Most automatic transmissions have multiple planetary gear arrangements. Through clutches, the ring gear, the sun gear and planetary pinion gears are capable of outputting several gear ratios. The three types of gears can mesh to produce a reduction: where the output speed is slower than the input speed; or an overdrive: where the output speed is faster than the input speed. For example, holding the ring gear stationary while the sun gear engages with the planetary carrier produces a reduction. However, if the sun gear is held stationary while the ring and planetary pinion gears mesh an overdrive is the output. Finally, if the driver wants to reverse the vehicle, the planetary pinion gears can be held stationary to support the gear reduction while the direction of rotation is reversed. The gears in the arrangement are typically helical—cut at an angle for progressive engagement—to provide a more smooth and quiet operation than spur gears.
The sun gear and sun gear shaft go through complex manufacturing processes to meet their respective design and fatigue life requirements. For example, the sun gear shaft must be designed with the appropriate size and material hardness to withstand at least 250,000 torque cycles at its maximum torque level. Moreover, the sun gear requires even more extensive material hardening to meet its pitting and bending fatigue strength requirements. If the sun gear and its shaft are integral, the heat treatment process required for the sun gear results in significant distortion of the sun gear shaft; post-heat-treatment straightening and machining are required to compensate. A more manufacturable—thus economical—design entails having the sun gear separable from the sun gear shaft during their respective manufacturing processes thereby simplifying their overall manufacturing process and reducing the need for post-heat-treatment straightening and machining. However, reconnecting the parts for operation requires a connecting mechanism, like splines, which can create the need for additional radial packaging space over that of an integral sun gear and sun gear shaft.
An electro-mechanical transmission is described and commonly assigned U.S. Ser. No. 10/946,915, Schmidt et al., filed Sep. 22, 2004, entitled “Two-Mode Compound-Split, Hybrid Electro-Mechanical Transmission Having Four Fixed Ratios,” assigned to General Motors Corporation and hereby incorporated by reference in its entirety.